A process of visualizing an image information through electrostatic images, such as an electrophotographic process, etc., has been utilized in various fields at present. In an electrophotographic process, an electrostatic image is formed on a photoreceptor by electrostatic charging and light-exposure step, and the electrostatic latent image is developed by a developer containing a toner and visualized through a transferring step and a fixing step.
As the developer used in the process, a two-component developer composed of a toner and a carrier and a one-component developer using a magnetic toner or a non-magnetic toner singly are known. For producing a toner, a knead-grinding method of melt-kneading a thermoplastic resin together with a pigment, a charge-controlling agent, and a releasing agent such as a wax, and after cooling, finely grinding and classifying the kneaded mixture to obtain a toner is usually used. In the toner, if necessary, inorganic or organic fine particles are added to the surfaces of the toner particles for the purpose of improving the fluidity and the cleaning property thereof. By these methods, a considerably excellent toner can be produced but they have following various problems.
That is, in an ordinary knead-grinding method, the form and the surface structure of the toner produced are a random-form and because the form thereof is delicately changed according to the grinding property and the condition of the grinding step, it is difficult to control the form and the surface structure of the toner. Also, in the knead-grinding method, there is a restriction on the range of selecting the material. Practically, it is preferred that a resin coloring agent dispersion is sufficiently brittle and can be finely ground by an ordinary grinder but when a resin coloring agent dispersion is brittle, it sometimes happens that a fine powder is generated from the toner by receiving a mechanical shearing force in a developing apparatus and the form of the toner is changed. In a two-component developer, the fine power generated is fixed to the surfaces of carriers to accelerate the deterioration of the developer and in a one-component developer, scattering of the toner occur by enlarging the particle size distribution, the developing property is lowered by the change of the toner form, and the image quality becomes liable to be deteriorated.
Also, in the toner containing therein a large amount of a releasing agent such as a wax, etc., by the combination with a thermoplastic resin, the resin frequently gives an influence on the exposure of the releasing agent on the surface of the toner. Particularly, in the combination of the resin which has an increased elasticity by high-molecular weight components and is hard to be ground to some extent and a wax, which is brittle, such as polyethylene, polyethylene is frequently exposed on the surface of the toner. The toner is advantageous for the releasing property at fixing and cleaning of the untransferred toner from a photoreceptor but the polyethylene on the surface layer of the toner is easily transferred onto a developing roll, a photoreceptor, a carrier, etc., to stain them and to lower the reliability.
Furthermore, when the form of a toner becomes a random form, even by adding a fluidity assistant, the fluidity of the toner cannot sufficiently ensured, a fine powder transfers to the concaved portions of the toner by receiving the mechanical shearing force during use to lower the fluidity of the toner with the passage of time, and also the fluidity assistant is embedded in the inside of the toner to deteriorate the developing property, the transferring property and the cleaning property. Also, when the toner recovered by cleaning is returned to the developing apparatus and used again, the image quality is liable to be lowered. For preventing the occurrences of these troubles, the amount of the fluidity assistant is increased, black sports are formed on the photoreceptor and also scattering of the assistant particles occurs.
As described above, in an electrophotographic process, for stably keeping the performance of a toner under various mechanical stresses, it is important that the exposure of a releasing agent onto the surface of the toner, the surface hardness of the toner is increased without reducing the fixing property of the toner, the both the mechanical strength of the toner and the sufficient charging property and fixing property of the toner are improved.
Furthermore, recently, the requirement for increasing the image quality has been increased and particularly, in the formation of color images, for realizing the images of a high brilliance, the tendency of small-sizing of toner is remarkable. However, when a toner is simply small-sized in a conventional particle size distribution of the toner, by the existence of the toner of the fine particle size, staining of a carrier and a photoreceptor and scattering to the toner become severe, and thus it is difficult to simultaneously realize the high image quality and the high reliability. For solving the problem, it becomes important that the particle distribution of the toner is sharpened and small-sizing of the toner is made possible.
Recently, in a digital full-color copying machine and printer, after color-separating a color image original by each of the filers of B (blue), R (red), and G (green), electrostatic images each composed of dots of the diameters in the range of from 20 to 70 .mu.m corresponding to the original are developed using each of the developers of Y (yellow), M (magenta), C (cyan), and Bk (black) by utilizing a subtractive mixing action. However, because in the digital full-color copying machine, etc., it is necessary to transfer large amounts of the developers as compared with a black and white copying machine, etc., of prior art and it is necessary to correspond to dots of smaller particle sizes, the uniform charging property, the durability, the toner strength, and the sharpness of the particle size distribution of the toner become more and more important. Also, considering the increase of the speed and the energy saving property of these copying machines, the low-temperature fixing property of the toner becomes more important. From these points, an aggregation unification method suitable for the production having a sharp particle size distribution and small particle sizes is excellent.
For the toner used for a full-color copying machine, etc., it is important that large amounts of the toners surely make color mixing and in this case, the improvement of the color reproducibility and the transparency of the OHP image become inevitable.
Also, as a means of controlling the form and the surface structure of a toner, a method of producing a toner by an emulsion polymerization aggregation method is proposed (Japanese Patent-Laid Open Nos. S63-282752 and H6-250439). According to the method, a resin fine particles dispersion is generally prepared, on the other hand, a coloring agent dispersion of a coloring agent dispersed in a solvent is formed, these dispersions are mixed to form aggregated particles having particle sizes corresponding to toner particle sizes, and the particles are formed into a coalescece by heating to product a toner. In the method, the form of the toner can be controlled to some extent and the charging property and the durability of the toner can be improved but it is difficult to control the dispersed state of the releasing agent and the coloring agent in the inside of the toner and in oil-less fixing, it is difficult to balance the releasing property of a sheet to be fixed and the transparency of the case of forming an OHP image.
In general, for the electrophotographic toner, a thermoplastic resin is used and to realize both low-energy fixing and a powder blocking property, it is proposed to optimum-control the rheology of the binder resin used for the toner and the glass transition point (Tg) of the binder resin (Japanese Patent Publication No. H2-37586, Japanese Patent Laid-Open Nos. H1-225967 and H2-235069).
Particularly, in a recent electrophotographic process, by the requirement of the progresses of the digital system and the high-speed system as described above, to cope with the far increase of the fixing speed, a binder resin having a lower glass transition temperature has been used.
However, when in the toner image containing the binder resin of this kind, heat of the temperature of the glass transition temperature of higher is applied to the image, there occurs a problem that the resin component of the image portion is melted and attaches to the back surface of the print or other prints to cause the defect of images, that is the problem of a document offset. Also, recently, double face printing has been increased but in this case because in the double face outputs, the images portions are inevitably in a contact state with each other, the occurrence of the image defects are more liable to occur than the case of one face output.
For improvement of the problem of the occurrence of the offset of these fixed output images, that is, the occurrence of the document offset, it is proposed that the both the offset resistance at fixing and the low-temperature fixing property and the transparency of the OHP image are realized, and also by curing the surface of the toner with a polyurethane resin and containing a adhesion resin together with a binder resin in the inside of the toner, both the offset resistance and the low-temperature fixing property are realized (Japanese Patent Laid-Open Nos. H4-186368, H2-101477, etc.).
However, these methods are effective for improving the document offset property, but because there is a tendency of stiffening the fixed images, when the fixed image is bent, an image defect becomes liable to occur. Also, because the melt viscosity of the toner is increased, the smoothness of the surface of the fixed image is liable to be reluctant to obtain and it sometimes happens that the surface gloss and the transparency of the OHP image are reduced.
Also, in general, for the purpose of preventing the occurrence of a low-temperature offset at fixing, a polyolefin-base wax is internally added to the toner as a releasing component and at the same time, a slight amount of a silicone oil is uniformly coated on the fixing roll to improve the high-temperature offset property. However, this method is undesirable because the silicone oil attaches to the output transfer material to cause an unpleasant sticky feeling occurs.
Thus, in Japanese Patent Laid-Open No. H5-61239, a toner for oil-less fixing containing a large amount of a releasing agent component in the toner is proposed. However, the addition of a large amount of a releasing agent can improve the releasing property but in this case a binder resin component is compatible with the releasing agent, oozing of the releasing agent becomes ununiform and the releasing property becomes unstable. Also, it sometimes happens that a free component of the releasing agent causes the hindrance of charging. Furthermore, in regard to the dispersibility of a pigment in the inside of the toner, by the interaction with the releasing agent, aggregates of the pigment are formed, which cause the problems of the hindrance of the transparency of the OHP image, the hindrance of coloring, etc.
On the other hand, for the purpose of improving the releasing property of a sheet to be fixed thereon, it has been proposed to add a crosslinking agent to the binder resin to improve the cohesive force of the binder resin is proposed (Japanese Patent Laid-Open Nos. S59-218459 and S59-218460). However, the method of simply adding a crosslinking agent to a binder resin increases the cohesive force of the binder resin itself, improves the releasing property of the toner, and makes it difficult to cause image defects (document offset) in the case of contacting imaged portions each other, but because the stiffness of the binder resin itself is increased, the bending resistance of the fixed image is lowered. Also, because the melt viscosity of the binder resin is increased, the smoothness of the surface of the fixed image is reduced, the gloss of the fixed image and the transparency of the OHP image are lowered, and the color mixing property in a full-color image is reduced.
Also, in Japanese Patent Laid-Open Nos. H4-69666 and H9-258481, a method of improving the apparent cohesive force of a binder resin by property adding thereto a high molecular amount component is proposed. In these methods, the cohesive force can be controlled to some extent by controlling the molecular weight and the glass transition temperature of the main binder resin component, but it is difficult to realize both the resistance to document offset (image defect) in the case of contacting imaged portions each other and the bending resistance.